Electric field/magnetic field sensors and methods of fabricating the same

ABSTRACT

An electric field sensor is obtained by directly forming an electrooptical film of Fabry-Perot resonator structure on a polished surface at a tip of an optical fiber by an aerosol deposition method.

TECHNICAL FIELD

This invention relates to electric field/magnetic field sensors andmethods of fabricating them and, in particular, relates to electricfield/magnetic field sensors each having high sensitivity/high spatialresolution for application to a very small region of an LSI chip/packageand methods of fabricating them.

BACKGROUND ART

Sensors and detection systems for detecting physical quantities such asan electric field and a magnetic field are disclosed in Patent Document1 (Japanese Unexamined Patent Application Publication (JP-A) No. Sho59-166873) and Patent Document 2 (Japanese Unexamined Patent ApplicationPublication (JP-A) No. Hei 2-28574).

FIG. 1 is a sectional view showing the structure of a conventionalhigh-spatial-resolution electric field sensor using an optical techniqueand FIG. 2 is a diagram showing an example of a detection system usingthe electric field sensor of FIG. 1.

Referring to FIG. 1, an electric field sensor 905 is bonded to the tipof an optical fiber 901 through an adhesive layer 906. The electricfield sensor 905 comprises a fine electrooptical crystal 907 serving asan electric field detection element and a dielectric multilayerreflective layer 908 formed on a bottom surface of the electroopticalcrystal 907 for reflecting light.

Referring to FIG. 2, the detection system comprises a continuous laserlight source 900, fiber amplifiers 902 and 911, a polarizationcontroller 903, an optical circulator 904, the electric field sensor 905provided over a circuit board 909 being an object to be measured, ananalyzer 910, a photodetector 912, optical fibers 901 connecting them toeach other, and a spectrum analyzer 913.

The electric field detection principle of this detection system will bebriefly described hereinbelow. Light emitted from the continuous laserlight source 900 is amplified by the fiber amplifier 902 and subjectedto control of its polarization plane by the polarization controller 903and then is incident on the electric field sensor 905 through theoptical circulator 904. The incident light on the electric field sensor905 is reflected by the dielectric multilayer reflective layer 908formed on the bottom surface of the electrooptical crystal 907 and thenis again returned into the optical fiber 901. Since the electroopticalcrystal 907 changes its refractive index depending on an electric fieldgenerated from the circuit board 909, the polarization state of thelaser light propagating in the crystal changes while being subjected tomodulation according to the intensity of the external electric field.The modulated light again passes through the optical circulator 904,then is converted into intensity-modulated light by the analyzer 910 andamplified by the fiber amplifier 911, and then is converted into anelectrical signal by the photodetector 912.

The electrical signal is detected by the spectrum analyzer 913 and apeak that occurs at that time is determined to be a signal caused by theexternal electric field. On the principle of this detection system, thesignal intensity differs depending on the intensity of the externalelectric field and, therefore, the electric field distribution isobtained by changing the position of the electric field sensor 905 overthe circuit board 909.

Incidentally, by replacing the electrooptical crystal 907 in FIG. 1 witha magnetooptical crystal, the system of FIG. 2 becomes a magnetic fielddetection system having high spatial resolution. The magnetic fielddetection principle in this case can be explained by replacing “electricfield” with “magnetic field” in the foregoing explanation of theelectric field detection principle.

As described above, the conventional electric field detection system ormagnetic field detection system having high spatial resolution ischaracterized by having the structure in which the microfabricatedelectrooptical crystal or magnetooptical crystal is bonded to the tip ofthe optical fiber 901.

An application region and spatial resolution of an electric fielddetection system or a magnetic field detection system are limited by thesize of an electrooptical crystal or a magnetooptical crystal and, asthe size decreases, the system can be applied to a smaller region andhas a higher spatial resolution. The spatial resolution is determinedbased on the volume of sensor light propagating in the crystal and, asthe volume of the sensor light decreases, the spatial resolutionincreases. For example, to describe a conventional magnetic field sensorin which a magnetooptical crystal is bonded to the tip of an opticalfiber, the magnetic field sensor having a 10 μm-class spatial resolutionis realized using the crystal having a plane size of 270 μm×270 μm and athickness of 11 μm.

However, with such a structure, it is difficult to realize a furtherreduction in size and a further increase in spatial resolution of asensor due to the limitation of the crystal microfabrication techniqueand, thus, it is not possible to provide a sensor applicable to a verysmall region of an LSI chip/package.

Further, in the case of the conventional type sensor, since the crystalis bonded to the tip of the optical fiber as described above, loss oflight is caused by the adhesive layer and this loss causes a reductionin sensitivity of the sensor and thus makes it difficult to detect avery small electric field or magnetic field generated from an LSI chipor the like.

It is an object of this invention to realize a sensor having highsensitivity and high spatial resolution while being smaller in size thanthe conventional electric field/magnetic field sensor, thereby providingthe sensor applicable to a very small region of an LSI chip/package.

DISCLOSURE OF THE INVENTION

This invention has been made based on the knowledge that it is effectiveto directly form an electrooptical layer or a magnetooptical layer inthe form of a thin film at the tip of an optical fiber for the purposeof realizing an electric field/magnetic field sensor having highsensitivity and high resolution.

An electric field sensor according to this invention is characterized inthat an electrooptical layer is directly formed at the tip of an opticalfiber. With this configuration, a reduction in thickness of theelectrooptical layer is enabled to thereby realize high resolution.Further, the interference effect can be utilized to thereby achieve anincrease in sensitivity.

Further, an electric field sensor according to this invention ischaracterized in that an electrooptical layer is directly formed at thetip of an optical fiber and a reflective layer is formed on the surfaceof the electrooptical layer. An electric field sensor according to thisinvention enables a further increase in sensitivity by directlylaminating, at the tip of an optical fiber, an electrooptical layer anda lower and an upper reflective layer so as to vertically sandwichtherebetween the electrooptical layer to thereby form a Fabry-Perotresonator.

It is preferable that a diameter d of the electrooptical layer satisfy arelationship of dc≦d≦dr with a diameter dc of a core of the opticalfiber and a diameter dr of a clad thereof.

Further, by setting a thickness t of the electrooptical layer to t≧1 μm,it is possible to increase the Q value of the Fabry-Perot resonator andthus to achieve an increase in sensitivity. The electrooptical layer ispreferably formed by a film forming method, particularly an aerosoldeposition method. According to the aerosol deposition method, it ispossible to form an electrooptical film having a thickness of 1 μm ormore and thus to enhance the sensitivity.

The composition of the electrooptical layer is one of lead zirconatetitanate, lanthanum-added lead zirconate titanate, barium titanate,strontium-added barium titanate, and tantalum-added potassium niobate.

A magnetic field sensor according to this invention is characterized inthat a magnetooptical layer is directly formed at the tip of an opticalfiber. With this configuration, a reduction in thickness of themagnetooptical layer is enabled to thereby realize high resolution.Further, the interference effect can be utilized to thereby achieve anincrease in sensitivity.

Further, a magnetic field sensor according to this invention ischaracterized in that a magnetooptical layer is directly formed at thetip of an optical fiber and a reflective layer is formed on the surfaceof the magnetooptical layer. A magnetic field sensor according to thisinvention enables a further increase in sensitivity by directlylaminating, at the tip of an optical fiber, a magnetooptical layer and alower and an upper reflective layer so as to vertically sandwichtherebetween the magnetooptical layer to thereby form a Fabry-Perotresonator.

It is preferable that a diameter d of the magnetooptical layer satisfy arelationship of dc≦d≦dr with a diameter dc of a core of the opticalfiber and a diameter dr of a clad thereof.

Further, by setting a thickness t of the magnetooptical layer to t≧1 μm,it is possible to increase the Q value of the Fabry-Perot resonator andthus to achieve an increase in sensitivity. The magnetooptical layer ispreferably formed by a film forming method, particularly an aerosoldeposition method. According to the aerosol deposition method, it ispossible to form a magnetooptical film having a thickness of 1 μm ormore and thus to enhance the sensitivity.

The magnetooptical layer is a ferrite having one of a garnet structure,a spinel structure, and a hexagonal structure. The magnetooptical layermay be a ferromagnetic film containing one of iron, nickel, and cobalt.

According to this invention, there is provided an electric field sensorfabrication method characterized by directly forming, at a tip of anoptical fiber, an electrooptical layer that changes its refractive indexdepending on an electric field. A reflective layer may be formed on thesurface of the electrooptical layer.

Further, according to this invention, there is provided an electricfield sensor fabrication method characterized by comprising the steps ofdirectly forming a first reflective layer at a tip of an optical fiber,directly forming, on the first reflective layer, an electrooptical layerthat changes its refractive index depending on an electric field, anddirectly forming a second reflective layer on the electrooptical layer.

In each of the foregoing fabrication methods, there is provided amagnetic field sensor fabrication method by forming, instead of theelectrooptical layer, a magnetooptical layer that changes its refractiveindex depending on a magnetic field.

Further, according to this invention, there are provided an electricfield detection system comprising the foregoing electric field sensorand a magnetic field detection system comprising the foregoing magneticfield sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of a conventionalelectric field sensor.

FIG. 2 is a block diagram showing the structure of an electric fielddetection system using the electric field sensor of FIG. 1.

FIG. 3 is a sectional view showing the structure of an electric fieldsensor according to a first embodiment of this invention.

FIG. 4 is a block diagram showing the structure of an electric fielddetection system using the electric field sensor of FIG. 3.

FIG. 5 is a photograph according to an SEM of an electric field sensoraccording to this invention.

FIG. 6 is a diagram showing the reflection spectra of the electric fieldsensor according to this invention and the conventional electric fieldsensor.

FIG. 7 is a diagram showing the electric field distribution according tothe electric field sensor of this invention and the electric fielddistribution according to the conventional electric field sensor.

FIG. 8 is a sectional view showing a second embodiment that can furtherenhance the sensor sensitivity in the electric field sensor according tothis invention.

FIG. 9 is a sectional view showing a third embodiment that can enhancethe sensor sensitivity more than the second embodiment of FIG. 8 in theelectric field sensor according to this invention.

FIG. 10 is a diagram showing the PZT film thickness dependence of thereflection spectrum of the electric field sensor according to thisinvention.

FIG. 11 is a sectional view showing the structure of a magnetic fieldsensor according to this invention.

FIG. 12 is a block diagram showing the structure of a magnetic fielddetection system using the magnetic field sensor of FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of this invention will be described with reference to thedrawings.

FIG. 3 is a sectional view showing the structure of an electric fieldsensor according to the first embodiment of this invention and FIG. 4 isa block diagram showing the structure of an electric field detectionsystem using the electric field sensor of FIG. 3.

Referring to FIG. 3, an electric field sensor 105 comprises a core layer106 and a clad layer 107 surrounding the core layer 106, both forming anoptical fiber 101, and an electrooptical layer 108 formed at the tip ofthe optical fiber 101. The tip of the optical fiber 101 is flattened bypolishing and the electrooptical layer 108 is directly formed on thepolished surface.

Referring to FIG. 4, the electric field detection system comprises acontinuous laser light source 100, fiber amplifiers 102 and 112, apolarization controller 103, an optical circulator 104, the electricfield sensor 105 provided over a circuit board 109 being an object to bemeasured, an analyzer 111, a photodetector 113, the optical fibers 101connecting them to each other, and a spectrum analyzer 114.

Laser light emitted from the continuous laser light source 100 isamplified by the fiber amplifier 102 and subjected to control of itspolarization plane by the polarization controller 103 and then isincident on the electric field sensor 105 through the optical circulator104. Since the electrooptical layer 108 changes its refractive indexdepending on an electric field generated from the circuit board 109, thepolarization state of reflected laser light 110 changes. The reflectedlaser light 110 passes through the optical circulator 104, then isconverted into light indicating its polarization state by the analyzer111 and amplified by the fiber amplifier 112, and then is converted intoan electrical signal by the photodetector 113. The converted electricalsignal is analyzed by the spectrum analyzer 114.

Since the resolution of the electric field sensor 105 for an amount ofchange in polarization state is determined by the thickness of theelectrooptical layer 108, it is desirable that the electrooptical layer108 be thin. On the other hand, the output of the electric field sensor105 is the product of an electrooptical coefficient representing anamount of change in refractive index by an electric field and thethickness of the electrooptical layer 108. Therefore, in order tosimultaneously satisfy high resolution and high output, it is importantto cause the electrooptical layer 108 serving as a sensor portion tohave an interference effect to thereby increase the apparent opticalpath length.

The conventional example shown in FIG. 1 is configured such that thebulk electrooptical member is thin-layered and bonded to the tip of theoptical fiber. However, with such a configuration, it is difficult toachieve parallelism with the end surface of the optical fiber and thusis not possible to obtain a sufficient interference effect. Further, thebulk member can be thin-layered to about 10 μm at most due to theprocessing limitation and thus the resolution cannot be enhanced.

In this embodiment, the electric field sensor having high sensitivityand high resolution has been realized by directly forming theelectrooptical layer 108 in the form of a thin film at the tip of theoptical fiber 101.

The electrooptical layer 108 was formed by an aerosol deposition methodthat forms a compact by pulverizing an ultrafine particle brittlematerial with the application of a mechanical impact force thereto andbonding them together. The film thickness was 9 microns. The filmformation was carried out using Pb(Zr_(0.6)Ti_(0.4))O₃ (hereinafterreferred to as PZT) as a material powder under the conditions that acarrier gas was oxygen, the incident angle between a nozzle and asubstrate was 10 degrees, the gas flow rate was 12 liters/min, thedistance between the nozzle and the substrate was 5 mm, the film formingrate was 0.8 μm/min, and the vibration frequency of a vibrator was 250rpm.

After the film formation, the electrooptical effect of theelectrooptical layer 108 was expressed by heat treatment in theatmosphere at 600° C. for about 15 minutes. Further, polarization wascarried out under the application of an electric field of about 100kV/cm at 200° C. The primary electrooptical coefficient r₃₃ was 200pm/V.

FIG. 5 shows a photograph according to an SEM of a PZT film 202 formedat an end of an optical fiber 201 by the aerosol deposition method. Itis seen that the PZT film 202 is formed to a thickness of 9 micronstightly to the end of the optical fiber 201. The aerosol depositionmethod has a feature of making it possible to form a thick film of acomplex oxide such as PZT in a short time.

After the heat treatment, the electrooptical layer 202 (108) waspolished to a thickness of 7 microns so as to be flattened in order toremove unevenness of the film surface thereof.

FIG. 6 shows the wavelength dependence of the reflection amount afterthe flattening of the film surface of the electrooptical layer 202(108). 301 denotes the reflection spectrum of this invention, wherein amodulation factor of about 30 dB is obtained. This represents that theelectrooptical layer formed according to this invention achieves a largeresonance structure, which is excellent as an EO sensor. For comparison,the reflection spectrum of the conventional EO sensor using the EOcrystal is shown at 302. In this conventional example, the modulationfactor is about 2 dB and thus it cannot be said that the sufficientresonance structure is obtained.

In the foregoing description, the composition of the electroopticallayer has been described in the case of PZT. However, the composition isnot limited thereto and, for example, may be added with La.

Further, other than the lead zirconate titanate based material, bariumtitanate, strontium-substituted barium titanate, tantalum-substitutedpotassium niobate, or the like having a large electrooptical effect isalso an effective material.

In this invention, the aerosol deposition method is used for forming theelectrooptical layer 108, which is one of the features of thisinvention. The reason therefor is as follows.

One of the objects of this invention is to provide an electric fieldsensor having high sensitivity and high resolution. For this purpose, itis important to directly form the electrooptical layer 108 in the formof a thin film at the tip of the optical fiber 101. On the other hand,in order to obtain a high interference effect, the thickness of theelectrooptical layer 108 is preferably 1 μm or more. Under the currenttechnique, it is quite difficult for even a sputtering method or asol-gel method to realize a 1 μm ferroelectric transparent film on aglass, a plastic, a resin containing a polymer, or a dielectric of anarbitrary composition, while, the aerosol deposition method can easilyrealize it.

It is important that a diameter d of the electrooptical layer 108satisfy a relationship of dc≦d≦dr with a diameter dc of the core 106 ofthe optical fiber 101 and a diameter dr of the clad 107 thereof. If thediameter d is no greater than dc, incident laser light is scattered andthus it is not possible to obtain a sufficient reflection light amount.Further, it is difficult for the film forming technique to form thediameter d no less than the diameter dr of the clad 107.

FIG. 7 shows the results of measuring the electric field distributionusing the electric field sensor 105 of this embodiment and the resultsof measuring the electric field distribution using the conventionalelectric field sensor, over three-line meandering wiring with a linewidth/space of 5 μm. A 10 MHz, 15 dBm signal was applied to themeandering wiring. The distributions of FIG. 7 were each obtained bydisposing the electric field sensor at a position of 10 μm above thewiring and scanning it at a pitch of 1 μm in a direction crossing thewiring. In the case of the conventional sensor, electric field peaksthat should be observed between the adjacent lines were indistinct,while, the electric field peaks were distinctly observed with theapplication of the sensor of this invention. That is, FIG. 7 is oneexample showing that the electric field sensor of this invention has ahigher spatial resolution than the conventional electric field sensor.

FIG. 8 is a sectional view showing the structure of an electric fieldsensor according to the second embodiment of this invention and showsthe structure that can further enhance the sensor sensitivity. In thisembodiment, a dielectric multilayer reflective film 504 is added to thesurface of an electrooptical layer 508 equivalent to the electroopticallayer 108 of the electric field sensor according to the firstembodiment.

In FIG. 8, an electric field sensor 505 is fabricated by forming theelectrooptical layer 508 at the tip of an optical fiber 501 comprising acore layer 506 and a clad layer 507 surrounding it. The tip of theoptical fiber 501 is flattened by polishing and the electrooptical layer508 is directly formed on the polished surface. The structure andfabrication method of the electrooptical layer 508 are the same as thosein the first embodiment.

The dielectric multilayer reflective layer 504 was formed on theflattened electrooptical layer 508 by an ion plating method. Thedielectric multilayer reflective layer 504 was formed by alternatelyforming SiO₂ films each having a thickness of 303 nm and Ta₂O₅ filmseach having a thickness of 186 nm. Film thickness control was carriedout by opening/closing a shutter over a deposition source whilemeasuring the optical spectrum using a monitor during the filmformation. Using the dielectric multilayer film reflective layer 504,the interference effect can be enhanced while reducing the influence onan electric field being measured.

FIG. 9 is a sectional view showing the structure of an electric fieldsensor according to the third embodiment of this invention and shows thestructure that can further enhance the sensor sensitivity. In thisembodiment, the Fabry-Perot resonator structure is formed by directlylaminating, at the tip of an optical fiber 601, a lower dielectricmultilayer film reflective layer 603 and an upper dielectric multilayerfilm reflective layer 604 so as to vertically sandwich therebetween anelectrooptical layer 608 equivalent to the electrooptical layer 108 ofthe electric field sensor according to the first embodiment.

In an electric field sensor 605, the lower dielectric multilayer filmreflective layer 603 is formed at the tip of the optical fiber 601comprising a core layer 606 and a clad layer 607 surrounding it. The tipof the optical fiber 601 is flattened by polishing and the lowerdielectric multilayer film reflective layer 603 is directly formed onthe polished surface.

The lower dielectric multilayer reflective layer 603 was formed by anion plating method. The lower dielectric multilayer reflective layer 603was formed by alternately forming SiO₂ films each having a thickness of303 nm and Ta₂O₅ films each having a thickness of 186 nm. Film thicknesscontrol was carried out by opening/closing a shutter over a depositionsource while measuring the optical spectrum using a monitor during thefilm formation. The electrooptical layer 608 and the upper dielectricmultilayer film layer 604 were formed on the lower dielectric multilayerreflective layer 603. The structures and fabrication methods of theelectrooptical layer 608 and the upper dielectric multilayer film layer604 are the same as those in the second embodiment.

FIG. 10 shows the electrooptical layer PZT film thickness dependence ofthe reflectance spectrum of the third embodiment. As the PZT filmthickness increases, the half width of the resonance peak, where thereflectance is lowered, decreases. Since high-sensitivity sensingrequires a Q value of 1000 or more, it is necessary that the PZT filmthickness be 1 μm or more.

FIG. 11 is a sectional view showing the structure of a magnetic fieldsensor according to this invention and FIG. 12 is a block diagramshowing the structure of a magnetic field detection system using themagnetic field sensor of FIG. 11.

Referring to FIG. 11, a magnetic field sensor 805 comprises a core layer806 and a clad layer 807 surrounding the core layer 806, both forming anoptical fiber 801, and a magnetooptical layer 808 formed at the tip ofthe optical fiber 801. The tip of the optical fiber 801 is flattened bypolishing and the magnetooptical layer 808 is directly formed on thepolished surface.

Referring to FIG. 12, the magnetic field detection system comprises acontinuous laser light source 800, fiber amplifiers 802 and 812, apolarization controller 803, an optical circulator 804, the magneticfield sensor 805 provided over a circuit board 809 being an object to bemeasured, an analyzer 811, a photodetector 813, the optical fibers 801connecting them to each other, and a spectrum analyzer 814.

Laser light emitted from the continuous laser light source 800 isamplified by the fiber amplifier 802 and subjected to control of itspolarization plane by the polarization controller 803 and then isincident on the magnetic field sensor 805 through the optical circulator804.

Since the magnetooptical layer 808 changes the Faraday rotation angledepending on a magnetic field generated from the circuit board 809, thepolarization state of reflected laser light 810 changes. The reflectedlaser light 810 passes through the optical circulator 804, then isconverted into light indicating its polarization state by the analyzer811 and amplified by the fiber amplifier 812, and then is converted intoan electrical signal by the photodetector 813. The converted electricalsignal can be analyzed by the spectrum analyzer 814.

Since the resolution of the magnetic field sensor 805 for an amount ofchange in polarization state is determined by the thickness of themagnetooptical layer 808, it is desirable that the magnetooptical layer808 be thin. On the other hand, the output of the magnetic field sensor805 is the product of the Faraday rotation angle and the thickness ofthe magnetooptical layer 808. Therefore, in order to simultaneouslysatisfy high resolution and high output, it is important to cause themagnetooptical layer 808 serving as a sensor portion to have aninterference effect to thereby increase the apparent optical pathlength. The conventional example is configured such that the bulkmagnetooptical member is thin-layered and bonded to the tip of theoptical fiber, but it is difficult to achieve parallelism with the endsurface of the optical fiber and thus is not possible to obtain asufficient interference effect. Further, the bulk member can bethin-layered to about 10 μm at most due to the processing limitation andthus the resolution cannot be enhanced.

In this embodiment, the high-sensitivity, high-resolution magnetic fieldsensor has been realized by directly forming the magnetooptical layer808 in the form of a thin film at the tip of the optical fiber 801.

The magnetooptical layer 808 was formed by an aerosol deposition methodthat forms a compact by pulverizing an ultrafine particle brittlematerial with the application of a mechanical impact force thereto andbonding them together. The film thickness was 4000 nm. The filmformation was carried out using Bi-substituted YIG garnet as a materialpowder under the conditions that a carrier gas was oxygen, the incidentangle between a nozzle and a substrate was 30 degrees, the gas flow ratewas 8 liters/min, the distance between the nozzle and the substrate was5 mm, the film forming rate was 1.0 μm/min, and the vibration frequencyof a vibrator was 250 rpm.

After the film formation, the magnetooptical effect of themagnetooptical layer 808 was expressed by heat treatment in theatmosphere at 600° C. for about 15 minutes. The Faraday rotation anglewas 7 deg/μm. After the heat treatment, the magnetooptical layer 808 waspolished to a thickness of 3600 nm so as to be flattened in order toremove unevenness of the film surface thereof.

In the foregoing description, the magnetooptical layer has beendescribed in the case of Bi-substituted YIG garnet. However, thecomposition is not limited thereto and, for example, may be added withCe.

Further, other than the YIG garnet based material, a ferrite or the likehaving either of a spinel structure and a hexagonal structure with alarge magnetooptical effect is also an effective material.

In this invention, the aerosol deposition method is used for forming themagnetooptical layer, which is one of the features of this invention.The reason therefor is as follows.

One of the objects of this invention is to provide a magnetic fieldsensor having high sensitivity and high resolution. For this purpose, itis important to directly form a magnetooptical layer in the form of athin film at the tip of an optical fiber. On the other hand, in order toobtain a high interference effect, the thickness thereof is preferably 1μm or more. Under the current technique, it is impossible for even asputtering method or a sol-gel method to realize a 1 μm ferromagnetictransparent film on a glass, a plastic, a resin containing a polymer, ora dielectric of an arbitrary composition, while, only the aerosoldeposition method makes it possible.

It is important that a diameter d of the magnetooptical layer 808satisfy a relationship of dc≦d≦dr with a diameter dc of the core of theoptical fiber 801 and a diameter dr of the clad thereof. If the diameterd of the magnetooptical layer 808 is no greater than the diameter dc ofthe core, incident laser light is scattered and thus it is not possibleto obtain a sufficient reflection light amount. Further, it is difficultfor the film forming technique to form the magnetooptical layer 808 tohave a diameter no less than the diameter dr of the clad.

Further, as the magnetooptical layer 808, it is possible to use aferromagnetic film in the form of a very thin layer containing one ofiron, nickel, and cobalt.

A magnetic field sensor according to this invention is not limited tothe example of FIG. 11, i.e. the same effect is achieved by replacingthe electrooptical film of each of the electric field sensors accordingto the first to third embodiments with a magnetooptical film. That is, amultilayer reflective layer may be formed on the surface of themagnetooptical layer 808 in the magnetic field sensor 805 of FIG. 11.

Further, a magnetic field sensor according to this invention may have astructure in which a first multilayer reflective layer and a secondmultilayer reflective layer are laminated so as to vertically sandwichtherebetween the magnetooptical layer 808 in the magnetic field sensor805 of FIG. 11.

As is clear from the foregoing description, according to this invention,there are provided electric field/magnetic field sensors each havinghigh sensitivity and high resolution.

1. A physical quantity sensor characterized by comprising an opticalfiber, and a physical optical layer directly formed at a tip of saidoptical fiber and changing its refractive index depending on a physicalquantity.
 2. A physical quantity sensor fabrication method characterizedby directly forming, at a tip of an optical fiber, a physical opticallayer that changes its refractive index depending on a physicalquantity.
 3. An electric field sensor characterized by comprising anoptical fiber, and an electrooptical layer directly formed at a tip ofsaid optical fiber and changing its refractive index depending on anelectric field.
 4. The electric field sensor according to claim 3,characterized by further comprising a reflective layer formed on asurface of said electrooptical layer.
 5. An electric field sensorcharacterized by comprising an optical fiber, a first reflective layerdirectly formed at a tip of said optical fiber, an electrooptical layerdirectly formed on said first reflective layer and changing itsrefractive index depending on an electric field, and a second reflectivelayer directly formed on said electrooptical layer.
 6. The electricfield sensor according to claim 3, characterized in that a relationshipof dc≦d≦dr is established among a diameter d of said electroopticallayer, a diameter dc of a core of said optical fiber, and a diameter drof a clad thereof.
 7. The electric field sensor according to claim 3,characterized in that a thickness t of said electrooptical layer is t≧1μm.
 8. The electric field sensor according to claim 3, characterized inthat said electrooptical layer is formed by a film forming method. 9.The electric field sensor according to claim 8, characterized in thatsaid electrooptical layer is formed by an aerosol deposition method. 10.The electric field sensor according to claim 3, characterized in that acomposition of said electrooptical layer is one of lead zirconatetitanate, lanthanum-added lead zirconate titanate, barium titanate,strontium-substituted barium titanate, and tantalum-substitutedpotassium niobate.
 11. A magnetic field sensor characterized bycomprising an optical fiber, and a magnetooptical layer directly formedat a tip of said optical fiber and changing its refractive indexdepending on a magnetic field.
 12. The magnetic field sensor accordingto claim 11, characterized by further comprising a reflective layerformed on a surface of said magnetooptical layer.
 13. A magnetic fieldsensor characterized by comprising an optical fiber, a first reflectivelayer directly formed at a tip of said optical fiber, a magnetoopticallayer directly formed on said first reflective layer and changing itsrefractive index depending on a magnetic field, and a second reflectivelayer directly formed on said magnetooptical layer.
 14. The magneticfield sensor according to claim 11, characterized in that a relationshipof dc≦d≦dr is established among a diameter d of said magnetoopticallayer, a diameter dc of a core of said optical fiber, and a diameter drof a clad thereof.
 15. The magnetic field sensor according to claim 11,characterized in that a thickness t of said magnetooptical layer is t≧1μm.
 16. The magnetic field sensor according to claim 11, characterizedin that said magnetooptical layer is formed by a film forming method.17. The magnetic field sensor according to claim 16, characterized inthat said magnetooptical layer is formed by an aerosol depositionmethod.
 18. The magnetic field sensor according to claim 11,characterized in that said magnetooptical layer is a ferrite having oneof a garnet structure, a spinel structure, and a hexagonal structure.19. The magnetic field sensor according to claim 11, characterized inthat said magnetooptical layer is a ferromagnetic film containing one ofiron, nickel, and cobalt.
 20. An electric field sensor fabricationmethod characterized by directly forming, at a tip of an optical fiber,an electrooptical layer that changes its refractive index depending onan electric field.
 21. The electric field sensor fabrication methodaccording to claim 20, characterized by forming a reflective layer on asurface of said electrooptical layer.
 22. An electric field sensorfabrication method characterized by comprising the steps of: directlyforming a first reflective layer at a tip of an optical fiber, directlyforming, on said first reflective layer, an electrooptical layer thatchanges its refractive index depending on an electric field, anddirectly forming a second reflective layer on said electrooptical layer.23. The electric field sensor fabrication method according to claim 20,characterized in that a relationship of dc≦d≦dr is established among adiameter d of said electrooptical layer, a diameter dc of a core of saidoptical fiber, and a diameter dr of a clad thereof.
 24. The electricfield sensor fabrication method according to claim 20, characterized inthat a thickness t of said electrooptical layer is t≧1 μm.
 25. Theelectric field sensor fabrication method according to claim 20,characterized in that said electrooptical layer is formed by a filmforming method.
 26. The electric field sensor fabrication methodaccording to claim 25, characterized in that said electrooptical layeris formed by an aerosol deposition method.
 27. The electric field sensorfabrication method according to claim 20, characterized in that acomposition of said electrooptical layer is one of lead zirconatetitanate, lanthanum-added lead zirconate titanate, barium titanate,strontium-substituted barium titanate, and tantalum-substitutedpotassium niobate.
 28. A magnetic field sensor fabrication methodcharacterized by directly forming, at a tip of an optical fiber, amagnetooptical layer that changes its refractive index depending on amagnetic field.
 29. The magnetic field sensor fabrication methodaccording to claim 28, characterized by forming a reflective layer on asurface of said magnetooptical layer.
 30. A magnetic field sensorfabrication method characterized by comprising the steps of: directlyforming a first reflective layer at a tip of an optical fiber, directlyforming, on said first reflective layer, a magnetooptical layer thatchanges its refractive index depending on a magnetic field, and directlyforming a second reflective layer on said magnetooptical layer.
 31. Themagnetic field sensor fabrication method according to claim 28,characterized in that a relationship of dc≦d≦dr is established among adiameter d of said magnetooptical layer, a diameter dc of a core of saidoptical fiber, and a diameter dr of a clad thereof.
 32. The magneticfield sensor fabrication method according to claim 28, characterized inthat a thickness t of said magnetooptical layer is t≧1 μm.
 33. Themagnetic field sensor fabrication method according to claim 28,characterized in that said magnetooptical layer is formed by a filmforming method.
 34. The magnetic field sensor fabrication methodaccording to claim 33, characterized in that said magnetooptical layeris formed by an aerosol deposition method.
 35. The magnetic field sensorfabrication method according to claim 28, characterized in that saidmagnetooptical layer is a ferrite having one of a garnet structure, aspinel structure, and a hexagonal structure.
 36. The magnetic fieldsensor fabrication method according to claim 28, characterized in thatsaid magnetooptical layer is a ferromagnetic film containing one ofiron, nickel, and cobalt.
 37. An electric field detection systemcharacterized by comprising the electric field sensor according to claim3.
 38. A magnetic field detection system characterized by comprising themagnetic field sensor according to claim 11.